SEOUL/BEIJING South Korea unleashed an ear-splitting propaganda barrage across its border with North Korea on Friday in retaliation for its nuclear test, while the United States called on China to end "business as usual" with its ally.

The broadcasts, in rolling bursts from walls of loudspeakers at 11 locations along the heavily militarised border, blared rhetoric critical of the Pyongyang regime as well as "K-pop" music. North Korea later responded with its own broadcasts.

Wednesday's nuclear test angered both the United States and China, which was not given prior notice, although the U.S. government and weapons experts doubt Pyongyang's claim that the device it set off was a hydrogen bomb.

China is North Korea's main economic and diplomatic backer, although relations between the Cold War allies have cooled in recent years.

China's Foreign Ministry urged North Korea to stick to its denuclearisation pledges and avoid action that would make the situation worse, but also said China did not hold the key to resolving the North Korean nuclear issue.

"Achieving denuclearisation of the Korean Peninsula and safeguarding the peninsula's peace and stability accords with all parties' mutual interests, is the responsibility of all parties, and requires all parties to put forth efforts," ministry spokeswoman Hua Chunying told a news briefing.

The North agreed to end its nuclear programme in international negotiations in 2005 but later walked away from the deal.

U.S. Secretary of State John Kerry said on Thursday that he had told Chinese Foreign Minister Wang Yi that China's approach to North Korea had not succeeded.

"APPROACH HAS NOT WORKED"

"China had a particular approach that it wanted to make, that we agreed and respected to give them space to implement that," Kerry told reporters after the phone call. "Today, in my conversation with the Chinese, I made it very clear that has not worked and we cannot continue business as usual."

In a call on Friday with his South Korean counterpart, Yun Byung-se, Wang said talks on the issue should be resumed as soon as possible, China's Foreign Ministry said.

South Korea's nuclear safety agency said it had found a minuscule amount of xenon gas in a sample from off its east coast, which could be the first chemical evidence of a nuclear test, but said more analysis and samples were needed to determine if it came from a nuclear test.

The presence of xenon would not indicate whether the blast was from a hydrogen device or not.

Seismic waves created by the blast were almost identical to those generated in North Korea's last nuclear test in 2013, Jeffrey Park, a seismologist at Yale University, wrote in a post on the Bulletin of the Atomic Scientists website, adding to scepticism about the hydrogen bomb claim.

Meanwhile, South Korea resumed its frontier broadcasts, which the isolated North has in the past threatened to stop with military strikes.

The last time South Korea deployed the loudspeakers, in retaliation for a landmine blast in August that wounded two South Korean soldiers, it led to an exchange of artillery fire.

The sound can carry 10 km (6 miles) into North Korea during the day and more than twice that at night, the South's Yonhap news agency reported.

BORDER PROPAGANDA

A male announcer could be heard from South Korea telling North Koreans that their leader Kim Jong Un and his wife wear clothes costing thousands of dollars. Another message said Kim's promises to boost both the economy and the nuclear programme were unrealistic.

The North's broadcasts were not clearly audible from the South and appeared intended to drown out those from the South, Yonhap said, citing a South Korean official.

As North Korea boosted troop deployments in front-line units, the South vowed to retaliate against any attack on its equipment, raised its military readiness to the highest level near the loudspeakers, cancelled tours of the Demilitarised Zone on the border, and also raised its cyberattack alert level.

In Washington, the North's actions appeared to have forged rare unity in the House of Representatives between Republicans and Democrats on tightening sanctions against North Korea.

Nancy Pelosi, the House Democratic leader, told reporters that Democrats would support a North Korea bill likely to be brought for a vote by Republicans next week. A congressional source said it was expected as soon as Monday.

But it is unclear how more sanctions will deter North Korea, which has conducted four nuclear tests since 2006.

The United States and South Korea are limited in their military options. Washington sent a pair of nuclear-capable B-2 stealth bombers over South Korea in a show of force after North Korea last tested a nuclear device in 2013.

North Korea responded then by threatening a nuclear strike on the United States.

A South Korean military official said Seoul and Washington had discussed the deployment of U.S. strategic weapons on the Korean peninsula, but declined to give details. Media said the assets could include B-2 and B-52 bombers, and a nuclear-powered submarine.